Process for the continuous treatment of vegetable fibers



Y I\ov. 6, 1956 F. G. L. BECKER 2,769,710

PROCESS FOR THE commuous TREATMENT OF VEGETABLE FIBERS Filed Jan. 12. 1954 INVENTOR Frederick GeorqeLucos Backer W United States Patent "cc PROCESS FOR THE CONTINUOUS TREATMENT OF VEGETABLE FIBERS Frederick George Lucas Becker, deceased, late of Hatch End, Middlesex, England, by Edward Goulding and Dorothy John, executors, London, England, assignors to Cellulose Development Corporation Limited, Hatch End, Middlesex, England, a British company Continuation of application Serial No. 119,699, October 2, 1949. This application January 12, 1954, Serial No.

2 Claims. (Cl. 92-7) This application is a continuation of application Serial No. 119,699, filed October 5, 1949, now abandoned.

This invention relates to processes and apparatus for the continuous treatment of vegetable fibres of the kind in which the fibres mixed with a treating liquid are arranged to descend through an upright tower and are withdrawn from the bottom of the tower and the invention is particularly concerned with the kind of process in which the treatment is carried out under elevated temperature and for this purpose the height of the tower is so selected that the pressure near the bottom corresponds to or is greater than the vapour pressure of the treating liquid at the required temperature. A process of this latter kind is described in British patent specification No. 536,339.

In order to facilitate the withdrawal of the treated material at the required rate and under considerable pressure which prevails at the bottom of the tower, means are described in this earlier specification for counterbalancing the pressure and for withdrawing the material at a required rate through the counterbalancing means.

An object of the present invention is to provide an improved control for the rate of withdrawal of the treated material from the bottom of the tower.

According to this invention, a process for the continuous treatment of vegetable fibres of the kind first'referred to above is characterised in that added liquid is pumped into the tower at or near thelower end thereof and in that the descending material, at the locality of the introduction of the added liquid, is strenuously agitated therewith and is then withdrawn from the bottom of the tower.

A method of continuously digesting vegetable fibrous material with a treating solution, such as caustic soda, sodium sulphite, sodium sulphide or other known digesting solutions consists in causing the vegetable fibres mixed with the treating solution to descend through an upright tower open or closed at thetop and heating the mixture during descent to a temperature exceeding 100 C., introducing water or spent digesting liquor at or near the bottom of the tower effecting agitation of said liquor with the vegetable material and withdrawing the resulting mixture, the height of which tower is sufiicient to enable pressure to be built up at or near the bottom thereof corresponding to the vapour pressure of the solution at said required temperature.

The upright tower may either be open or closed at the top and there are provided means for introducing the material to be treated and the treating liquid at the top of the tower, means for heating the material during its descent through the tower, means for introducing additional liquid at or near the bottom of the tower, means for agitating the material with the introduced liquid and means for withdrawing the resulting mixture from the bottom of the tower, the height of which tower above the locality of said liquid is sufficient to enable a pressure to be built up corresponding to the vapour pressure of the 2,769,710 Patented Nov. 6, 1956 liquid at a temperature at which the material requires to be treated.

The means for withdrawing the material mixed with the added liquid may comprise a regulating valve with or without an associated balancing column.

The aforesaid agitating means may comprise a rotor mounted to rotate about an upright axis and having outwardly radiating arms. Preferably, the added liquid is arranged to be introduced at a level opposite the uppermost radiating arm.

The lower part of the tower in which said agitating takes place is preferably of larger diameter than the upper part of the tower. The heating means may comprise jackets surrounding the upper part of the tower, the diameter of this'part of the tower being determined by the rate of heat transferred from the steam jackets to the interior and may be between 5 feet and 10 feet in diameter, while the lower part of the tower may be up to 20 feet in diameter, whereby the capacity of the tower, as a whole, is greatly increased. It is found in practice that the compact mass of fibrous material uniformly descends the smaller diameter portion of the tower and on reaching the bottom part thereof uniformly expands as it enters the wider portion. The material is retained in this wider portion while cooking continues, and preferably the walls of the larger portion are heat insulated by suitable lagging.

The following is a description of one form of apparatus suitable for digesting such material as cut grass, esparte grass, cereal straw, bagasse, bamboo etc. reference being made to the accompanying drawing, which is a diagramthrough nozzles (not shown) in proportion of about 3- or 4 of the liquid to 1 of the vegetable fibres. Steam may be introduced into the upper part of the tower through the pipe indicated at 12. Alternatively, or additionally, the material in the upper tower may be heated by steam jackets 13 which encircle the walls. As the material descends the tower it reacts with the treating solution and forms a compact mass, the temperature of which is progressively raised by means of the steam jackets around the walls until, in the case of a tower closed at the top, it reaches a temperature at the lower end corresponding to or slightly greater than the vapour pressure of the liquid at the lower end, or, in the case of a tower open at the top, it reaches a temperature slightly less than that corresponding to the vapour pressure. The upper part 13 of the tower opens into a base portion 14 of larger diameter and which may be up to 20 feet in diameter, whereby the capacity of the tower, as a whole, is greatly increased. The compact mass of fibrous material upon entering the base portion expands uniformly so as to fill it. The walls of this base portion may be well lagged so that cooking of the material continues as the material passes through it. It is thus possible to provide a digestion unit of larger capacity than was formerly possi ble, without greatly increasing the height of the tower. The base portion is conically shaped at its lower end, and located in this end is an agitator 16 comprising an upright shaft carrying a number of outwardly radiating arms. Dilution water is introduced at approximately the same level as the uppermost radiating arm of the agitator 16. The rate of flow of the dilution water is controlled by a suitable pump 19. The solid content of the material" above the agitatonmay be between 20% and Whereasafte-r the materia-l--ismixed-with-theliquid-added"- by the nozzles the solid content may be 6% to 2% or any desired lesser consistency whereby an aqueous suspension is for-med Which may be rernoved readily from the system through a regulating valve 17. The cooked fibrous material above the water-inlet 15 remains in the form of a compactsmass sothat the added-water doesnot travel more than at-foot or s0 abovethe inlet 1'5. In

the casewhere the added liquid is coldzlwater thishasthe effect oficooling thecooked material. If recirculated, spent or'bl'ack liquor fr'oma' subsequent washing process is employed a heatexchanger'may be providedfor cool ing recirculated black liquor.

It will" be appreciated, however, that other forms of agitators maybe employed. For example, the agitator might comprise aflat" d-isc provided-with anumber of vanes; In this case, the. disc'would'require to-be rotated ata-higher speed than the agitator first described and wouldbedisposed-at about the-samelevel as the noz zles 15. The diluted'pulp' in-such amarrangementwould be. extracted just below the disc andthe bottom of thetower, instead of-being conical' could -be' of dished-shape; To simplify extraction andtoavoid any tendency for the valve to choke; the'pressureoneither side of the regulating valve 17 may h'e"balanced b'y' connecting the outletside to an-upwardly-extendi'ng conduit 18' through which the'diluted material is passed'through the'pl'ant.

Therate of extraction ofthe material through thevalve 17"may be automatically controlled in-accordancewiththe power absorbed by the agitator. For example,- ifthe pulp becomes too thick; the agitator'willabsorb-increased power; which may be registered on*-an"- ammet'erin circuit onversel'y=, if the pulp becomes too diluted, the" maybe quite easily regulatedby'hanql-ii;vaccordance with tating; the mixture. near the bottom". of: the: tower? at a.

pressure which is at least equal to the vapor pressure of the treating liquid at the elevated temperature and which is solely providedby. the height-of'thezmaterial. in the tower and" continuously introducing diluting liquid at the-top. of the zone of agitation and varying the rater of" introduction: ofir diluting liquidi in accordance. with the consistency of the mixture.

2. A method of continuously digesting, vegetable fibrous material with a treating solution which consists in causing; the vegetable. fibersmixed'with a suitable reagent under elevatedtemperature to continuously. descend under gravity through an. upright. towerv thereby rendering the impurities in the fibers separable,v continuously agitating; the mixturenear the bottom. ofthe tower at apressure. whichisrat least equal to. the vapor pressure. of the treating liquid at the elevated temperature and which is sol'ely provided by the height of the material in the tower, measuring, the power. required foragitation of the mixture. and. introducing diluting,liquidinto the agitated mixture at a rate. which is varied in accordance. with the: power. requiredfor. agitation. of the-mixture whereby to maintain. the. mixture asv an aqueousv suspension which is readily) continuously discharged.

References Cited 'in the file. of thispatent UNITED STATES PATENTS Richter July 5, 1949 OTHER REFERENCES.

Swarr, et: al'.: Blowpit'. Consistency Control, TAPPI Seer, pp. 58-621 A'ugustz 10',- 1944'.

A: Kraft BrownvStocleRefining' at'Bathurst Power and- Paper' 60.: MilL. fnom.-the.Raper' Industry and Paper World foriMarch '1947,Ipp;. 1804; 1806; 1808', 

1. A METHOD OF CONTINUOUSLY DIGESTING VEGETABLE FIBROUS MATERIAL WITH A TREATING SOLUTION WHICH CONSISTS IN CAUSING THE VEGETABLE FIBERS MIXED WITH A SUITABLE REAGENT UNDER ELEVATED TEMPERATURE TO CONTINUOUSLY DESCEND UNDER GRAVITY THROUGH AN UPRIGHT TOWER THEREBY RENDERING THE IMPURITIES IN THE FIBERS SEPARABLE, CONTINUOUSLY AGITATING THE MIXTURE NEAR THE BOTTOM OF THE TOWER AT A PRESSURE WHICH IS AT LEAST EQUAL TO THE VAPOR PRESSURE OF THE TREATING LIQUID AT THE ELEVATED TEMPERATURE AND 